In computer systems, buses are used to transmit data between individual modules (containing a microprocessor, a memory, etc.). The data are transmitted over the bus in the form of a high voltage signal (V.sub.OH), which represents a binary 1, and a low voltage signal (V.sub.OL), which represents a binary 0. Various logic systems have adopted different values for V.sub.OH and V.sub.OL. For example, in the CMOS system V.sub.OH =5V and V.sub.OL =0V; and in the transistor transistor logic (TTL) system, V.sub.OH =2V and V.sub.OL =0.8V. In the latest system proposed, designated as backplane transceiver logic (BTL), V.sub.OH =2.1V and V.sub.OL =1.1V. Thus, over time, the voltage difference between V.sub.OH and V.sub.OL has narrowed from 5V in CMOS to 1V in BTL. Bringing V.sub.OH and V.sub.OL closer together allows for a more rapid transfer of data and a reduced noise level.
Since a system is generally based on TTL or CMOS logic, a "transceiver" is necessary to transmit the data to and receive the data from a BTL bus. Each module is associated with a transceiver. The transceiver includes two parts: a driver for delivering data from a module to the bus and a receiver for receiving data from the bus and delivering it to the module. A general block diagram of a BTL bus is illustrated in FIG. 1, which shows modules M.sub.1 -M.sub.n linked via transceivers T.sub.1 -T.sub.n to a BTL bus 10.
A conventional BTL driver 20 is illustrated in FIG. 2. Driver 20 includes a bipolar transistor 21 and a Schottky diode 22. A control circuit 23 receives an input from a module and controls the base of bipolar transistor 21. Turning transistor 21 on pulls bus 10 low and represents a binary zero; turning transistor 21 off leaves bus 10 at 2.1V and represents a binary 1.
There are several disadvantages with this arrangement. First, a significant amount of base current is required to drive transistor 21. Second, in order to turn transistor 21 off fast enough (for example, in 2 ns) current must be pulled from the base of transistor 21. This reduces the rise time of the pulses delivered by driver 20, and complicates control circuit 23. Third, it is difficult to control the rise time and the fall time of the output of driver 20. Finally, including a bipolar transistor in the driver creates a barrier to further integration since a BiCMOS process is more expensive than a CMOS process.
These problems are overcome in a BTL driver in accordance with this invention.